1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery and a battery module.
2. Description of the Related Art
A nonaqueous electrolyte secondary battery having a negative electrode containing a lithium metal, a lithium alloy, a lithium compound or a carbon material has been intensively researched and developed as a high energy density battery or high output density battery. To date, lithium ion batteries having a positive electrode containing LiCoO2 or LiMn2O4 as an active material and a negative electrode containing a carbon material for intercalating and deintercalating lithium have been put into practical use. For the negative electrode, it has been attempted to use a metal oxide or alloy instead of the carbon material.
The current collector of the negative electrode is generally made of copper foil. However, when a nonaqueous electrolyte secondary battery having a current collector made of copper foil is overdischarged, the dissolving reaction of the copper foil is promoted by potential elevation of the negative electrode, and the discharge capacity drops suddenly. Accordingly, the nonaqueous electrolyte secondary battery is provided with a protective circuit board for preventing overdischarge. However, the nonaqueous electrolyte secondary battery provided with a protective circuit board is not advantageous from the viewpoint of energy density.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2002-42889 discloses a nonaqueous electrolyte secondary battery comprising a negative electrode including a negative electrode current collector made of aluminum or aluminum alloy, and a negative electrode layer containing at least one negative electrode active material selected from the group consisting of a metal, an alloy and a compound for intercalating and deintercalating lithium. As a result, a nonaqueous electrolyte secondary battery enhanced in energy density and overdischarge cycle performance is realized.
However, since the negative electrode current collector made of aluminum foil or aluminum alloy foil proposed in Jpn. Pat. Appln. KOKAI Publication No. 2002-42889 is weak in strength, it may be broken when the pressing force is increased in order to enhance the negative electrode density. Therefore, according to the conventional negative electrode current collector, a thin negative electrode of high density cannot be obtained, thus and it has been difficult to increase the capacity of the nonaqueous electrolyte secondary battery.
Further, in the nonaqueous electrolyte secondary battery having the negative electrode containing the negative electrode current collector disclosed in the patent publication, when overdischarged in a high temperature environment, the negative electrode current collector reacts with the nonaqueous electrolyte, and is dissolved to deposit on the negative electrode. As a result, the current collecting performance of the negative electrode is lowered, and the electrode resistance increases, so that a sufficient charge and discharge cycle life cannot be obtained.